1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of the Related Art
In a sheet conveying apparatus provided to an image forming apparatus, when there is a change in sheet conveying velocity of particularly a roller nearest to a transferring portion for transferring an image onto a sheet, there is a possibility of occurring color drift, resulting in image deterioration.
In a conventional technique, in order to stabilize the sheet conveying velocity of the roller nearest to the transferring portion, sheet conveying velocities of rollers are made different from each other, thereby stabilizing the sheet conveying velocity of the roller nearest to the transferring portion (see Japanese Patent Application Laid-Open No. 2002-249258).
A structure of the conventional sheet conveying apparatus is illustrated in FIG. 7.
When a sheet S1 is conveyed by a sheet feeding roller 101 from a sheet feeding cassette 100 one at a time, the sheet S1 is conveyed successively by an intermediate conveying roller pair 102 and a registration roller pair 103. Then, the sheet S1 is attracted by an attracting roller 104 to a conveyor belt 105, and is conveyed to a transferring portion 106.
In FIG. 7, a sheet conveying velocity V1 of the attracting roller 104 and the conveyor belt 105, a sheet conveying velocity V2 of the registration roller pair 103 and a sheet conveying velocity V3 of the intermediate conveying roller pair 102 have a relationship of V1>V3>V2. A conveying guide 107 is provided between the intermediate conveying roller pair 102 and the registration roller pair 103.
In the case where the sheet conveying velocities are set in the above-mentioned manner, when the sheet S1 is nipped by the intermediate conveying roller pair 102, the sheet S1 is conveyed by the intermediate conveying roller pair 102 at the sheet conveying velocity V3. Next, when the sheet S1 reaches the registration roller pair 103, the sheet S1 is conveyed by the registration roller pair 103 at the sheet conveying velocity V2.
When the sheet S1 reaches the registration roller pair 103, a part of the sheet S1 is still conveyed by the intermediate conveying roller pair 102 at the sheet conveying velocity V3. Therefore, due to the relationship of V3>V2, the sheet S1 is bent between the intermediate conveying roller pair 102 and the registration roller pair 103, thereby generating a loop. That is, when the sheet S1 reaches the registration roller pair 103, the sheet S1 is conveyed by the registration roller pair 103 at the sheet conveying velocity V2 while forming the loop.
Next, when the sheet S1 conveyed by the registration roller pair 103 at the sheet conveying velocity V2 reaches the conveyor belt 105 via the attracting roller 104, the sheet S1 is conveyed by the attracting roller 104 and the conveyor belt 105 at the sheet conveying velocity V1 to the transferring portion 106. When the sheet S1 reaches the conveyor belt 105, a part of the sheet S1 is still conveyed by the registration roller pair 103 at the sheet conveying velocity V2. Therefore, due to the relationship of V1>V2, the sheet S1 is pulled between the attracting roller 104 and the registration roller pair 103.
When the sheet S1 is pulled as described above, because a drive portion of the registration roller pair 103 includes a one-way clutch mechanism, the registration roller pair 103 rotates along with the sheet S1. That is, when the sheet S1 is conveyed by the conveyor belt 105, the sheet conveying velocity of the registration roller pair 103 becomes the V1.
Meanwhile, when the sheet conveying velocity of the registration roller pair 103 becomes the V1, due to the relationship of V1>V3, the loop generated between the intermediate conveying roller pair 102 and the registration roller pair 103 due to the relationship of V3>V2 gradually becomes smaller. When the loop is eliminated, there is a fear of the sheet S1 being pulled.
In this manner, when the sheet S1 is pulled, because a one-way clutch mechanism is also provided to a drive portion of the intermediate conveying roller pair 102, the intermediate conveying roller pair 102 rotates integrally with the sheet S1. When the intermediate conveying roller pair 102 rotates integrally with the sheet S1, the sheet conveying velocity of the intermediate conveying roller pair 102 becomes the V1.
As descried above, when the sheet S1 is conveyed by the conveyor belt 105, the sheet conveying velocity of the intermediate conveying roller pair 102 and the registration roller pair 103 becomes the V1. As a result, the whole sheet conveying velocity is the constant sheet conveying velocity V1, thereby the sheet can be conveyed to the transferring portion 106 at the stable sheet conveying velocity V1.
In recent years, downsizing of image forming apparatuses is achieved. Accordingly, in the conventional image forming apparatus, a sheet conveying path is curved. However, in the case where the sheet conveying path is curved, when a sheet is pulled as described above so as to be stably conveyed to the transferring portion in the conventional sheet conveying apparatus, the pulled sheet is brought into contact with the conveying guide constituting the sheet conveying path in some cases.
When the sheet is brought into contact with the conveying guide, a sliding resistance of the sheet increases, thereby applying a load to conveying rollers immediately before transfer. As a result, when multiple sheets are conveyed, the conveying rollers immediately before the transfer are worn. Accordingly, the sheets cannot be continuously conveyed to the transferring portion at the stable conveying velocity.
As a separation portion for separating the sheets one by one fed from a sheet feeding cassette 100 by a sheet feeding roller 101, there may be adopted a structure in which back tension is large. In this case, when the sheet conveying velocity of the conveying rollers immediately before the sheet transfer is set to be fastest, the sheet is pulled from the separation portion when the sheet is conveyed by the conveying rollers immediately before the transfer. Therefore, a large force is required for the conveying rollers immediately before the transfer. Accordingly, occurrence of slip or wear of the rollers is promoted.
In the case where the sheets cannot be continuously conveyed to the transferring portion at the stable conveying velocity, or where the slip or the wear of the rollers is caused, color drift occurs in the transferring portion and stable image formation onto the sheets cannot be performed.
Japanese Patent Application Laid-Open No. 2002-362776 also discloses that a relationship among the conveying velocity V1 of the attracting roller 104, the conveying velocity V2 of the registration roller pair 103, and the conveying velocity V3 of the intermediate conveying roller pair 102 is V1>V3>V2. Japanese Patent Application Laid-Open No. 2002-362776 discloses that, as a reason for setting the conveying velocities to V1>V3>V2, when the conveying velocities are set to V1<V3, due to a difference in velocity between the intermediate conveying roller pair 102 and the registration roller pair 103, a loop existing upstream of the registration roller pair 103 becomes larger and larger. As a result, when a size of the loop existing upstream of the registration roller pair 19 exceeds a certain size, the loop causes the sheet to be pushed out from the registration roller pair 19. At last, the sheet is pushed out to the transferring portion, thereby causing a problem.
In order not to suffer the back tension by the separation portion with reliability, a loop can be formed in the sheet on the upstream of the rollers before the transfer. On the other hand, when a loop is formed in the sheet as described in Japanese Patent Application Laid-Open No. 2002-362776, due to stiffness of the sheet, the sheet is pushed in between the rollers before the transfer, so there is a fear of causing a problem in conveyance of the sheet.